It is common practice to "cement" oil and gas wells in unconsolidated or poorly consolidated formations. More particularly, oil and gas are commonly found in porous rock formations through which the oil and gas flows in production of a well. Often these porous formations are not consolidated or are poorly consolidated. Accordingly, it is normal to "case" a well, that is, line it with a continuous steel pipe, and then perforate the pipe in the vicinity of the productive part of the formation. However, this is often not adequate to provide "zonal isolation," that is, to seal the producing part of the well to the casing. Accordingly, it is common to "cement" the well, that is, pump a quantity of cement slurry between the casing of the well and the borehole. Typically the entire annular space between the casing and the borehole is thus filled with cement in this step. When the casing and the cement are subsequently perforated, substantial isolation of the particular zone of the formation to be produced is provided.
It is also well known that wells which have thus been cased, cemented and perforated may sometimes stop producing, or begin to produce undesirable fluids such as water, due to loss of isolation outside the perforation in the casing. Typically, the formation may erode outside the perforations, and collapse, blocking them; in some cases, the cement itself may erode away. In either case, the isolation provided by the cement between different layers of the formation may be lost. It is common in such circumstances to further cement the well and then to further perforate it. However, if a quantity of cement is simply pumped down the well at a high rate, it tends to flow farther into the formation than would be desirable, and to cause further difficulties. The flow rate at which this occurs varies to a considerable degree with formation strength; a loosely consolidated formation will typically be much more readily penetrated by the fast-flowing cement than a "harder" formation. The desired flow rate of the cement into the formation will also vary with pressure in the formation.
As far as the present inventor is aware, there is no particularly pertinent prior art tool addressing this exact problem. While valves placed in the down-hole tool assembly have been used to control the rate of flow of cement into formations, none of these have been positive flow control valves, merely spring loaded ball valves and the like, and none have been fully successful in controlling the rate of flow of cement into formations.
Accordingly, it is an object of the invention to provide a controlled rate well cementing tool, which can be used to deposit a quantity of cement in the well at the desired depth opposite the perforations, and to control the flow of the cement into the formation at a desired rate, in order to deposit it in a particularly desired way.